-I used simulation models to analyze the effect of different suites of life-history traits and mating rules on the correlation between ages of paired birds. If pair stability is high, the correlation between ages of mates may be strong even in the absence of active mate selection on the basis of age or experience. Even a low frequency of dissolved pairs substantially reduces the correlation between ages. For 9 species with known life-history traits, I simulated the extent of active mate selection required to produce the observed patterns of ages of mates. In only 1 species could the pattern of ages be explained in the absence of any active mate selection on the basis of the experience of the mate. In 5 species the results suggested there was strong active selection of experienced breeders. With the exception of the Adelie Penguin (Pygoscelis adeliae) and the Blue-eyed Shag (Phalacrocorax atriceps), observed patterns of ages of mates could be reproduced by simulation models assuming only that birds can distinguish experienced from inexperienced breeders. Received 20 April 1987, accepted 23 November 1987. THE members of pairs of breeding birds are frequently of similar age (Mills 1973, Coulson and Horobin 1976, Mills and Shaw 1980, Pugesek and Diem 1981, Ainley et al. 1983, Nisbet et al. 1984, Perrins and McCleery 1985, Shaw 1985). A common explanation for this similarity in age is nonrandom mate selection with respect to age or experience (Mills 1973, Nisbet et al. 1984, Shaw 1985). Because experienced breeders tend to have higher reproductive success (references given by Nisbet et al. 1984, Perrins and McCleery 1985, Shaw 1985), selection should exist for assortative mating. While a correlation between ages can be produced by the choice of particular mates, several other factors also could produce this pattern in the absence of active selection on the basis of age or experience. For example, suppose that birds enter the breeding population at the same age and tend to choose mates at the same time or location. If mortality rates are low and the birds rarely change mates, then the initial similarity in ages of mates would be preserved and the correlation between ages would be strong (Mills and Shaw 1980, Nisbet et al. 1984, Shaw 1985). Experimental tests of mate choice (e.g. Burley and Moran 1977) are the best means to assess 1 Present address: Resources for the Future, 1616 P Street N.W., Washington, D.C. 20036 USA. the extent of assortative mating. When experimental tests are impractical, the statistical analysis of patterns can be used to infer causation. Specifically, the correlation between ages of mated birds may be used as evidence for assortative mating if it significantly exceeds that expected under the null hypothesis of random mate choice. For example, Perrins and McCleery (1985) compared observed frequencies of pairings of new and and experienced Great Tits (Parus major) with frequencies expected under conditions of random mating and found no evidence of assortative mating despite an apparent excess of pairs of experienced breeders. In contrast, in studies of the Common Tern (Sterna hirundo; Nisbet et al. 1984), Blue-eyed Shag (Phalacrocorax atriceps; Shaw 1985), and Eurasian Sparrowhawk (Accipter nisus; Newton et al. 1981), the difference in the ages of mates differed significantly from patterns expected by random choice. The models used in these studies, however, assumed that birds were free to mate with any other bird; this assumption is false given the evidence for pair stability in these species. A different random model is needed as a null hypothesis against which to compare observed correlations. More generally, statistical studies of age correlations between mates would be strengthened by clarifying the influence of different life-history traits on the correlations expected under varying degrees of assortative mating. To address this issue I simulated the age correlations that result from dif278 The Auk 105: 278-285. April 1988 This content downloaded from 207.46.13.16 on Wed, 25 May 2016 06:12:47 UTC All use subject to http://about.jstor.org/terms April 1988] Age Correlations Between Mated Birds 279 ferent suites of life-history traits and mating rules. I used simulation models to analyze the effect of mortality rate, pair stability, and nonrandom mate choice on the correlation between ages of paired birds. The purpose was to demonstrate, for various values of survivorship and pair stability, the expected correlation between the ages of mated birds under various levels of assortative mating. I then determined the extent of nonrandom choice required to produce correlations observed in nine species for which field data are available. As a second test of the accuracy of the simulation model, I compared the distribution of age differences among older pairs in the simulation model with observed differences in field populations.